Tribology Series Grease Lubrication in Rolling Bearings Piet M. Lugt GREASE LUBRICATION IN ROLLING BEARINGS Tribology Series Bhushan Introduction to Tribology, 2nd Edition March 2013 Bhushan Principles and Applications to Tribology, 2nd March 2013 Edition Lugt Grease Lubrication in Rolling Bearings January 2013 Honary and Richter Biobased Lubricants and Greases: Technology April 2011 and Products Martin and Ohmae Nanolubricants April 2008 Khonsari and Applied Tribology: Bearing Design and April 2008 Booser Lubrication, 2nd Edition Stachowiak (ed) Wear: Materials, Mechanisms and Practice November 2005 Lansdown Lubrication and Lubricant Selection: A November 2003 Practical Guide, 3rd Edition Cartier Handbook of Surface Treatment and Coatings May 2003 Sherrington, Rowe Total Tribology: Towards an Integrated December 2002 and Wood (eds) Approach Kragelsky and Tribology: Lubrication, Friction and Wear April 2001 Stolarski and Tobe Rolling Contacts December 2000 Neale and Gee Guide to Wear Problems and Testing for October 2000 Industry GREASE LUBRICATION IN ROLLING BEARINGS Piet M. Lugt SKF, The Netherlands A John Wiley & Sons, Ltd., Publication This edition first published 2013 C 2013 John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For detailsof our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com. The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. 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ISBN 978-1-118-35391-2 (hardback) – ISBN 978-1-118-48396-1 (obook) – ISBN 978-1-118-48397-8 (epub) 1. Roller bearings–Lubrication. 2. Lubrication and lubricants. I. Title. TJ1071.L78 2013 621.89–dc23 2012031584 Cover photograph courtesy of SKF Maintenance Products A catalogue record for this book is available from the Library of Congress. ISBN: 978-1-118-35391-2 Typeset in 10/12pt Times by Aptara Inc., New Delhi, India Dedicated to: Marjo, Michiel and Marijn Contents Preface xvii Series Preface xix List of Abbreviations xxi 1 Introduction 1 1.1 Why Lubricate Rolling Bearings? 1 1.2 History of Grease Lubrication 2 1.3 Grease Versus Oil Lubrication 3 2 Lubrication Mechanisms 5 2.1 Introduction 5 2.2 Definition of Grease 6 2.3 Operating Conditions 6 2.4 The Phases in Grease Lubrication 7 2.5 Film Thickness During the Bleeding Phase 8 2.5.1 Ball Bearings 8 2.5.2 Roller Bearings 10 2.6 Feed and Loss Mechanisms During the Bleeding Phase 10 2.7 Film Thickness and Starvation (Side Flow) 11 2.8 Track Replenishment 12 2.9 Grease Flow 13 2.9.1 Non-Newtonian Rheology 14 2.10 Wall-Slip 15 2.11 Oxidation 16 2.12 EP Additives 16 2.13 Dynamic Behaviour 17 2.14 Grease Life 17 2.14.1 Temperature 18 2.14.2 Speed 19 2.14.3 Load 19 2.14.4 Bearing Type 20 2.14.5 Grease Type 20 2.14.6 Environment 21 viii Contents 3 Grease Composition and Properties 23 3.1 Base Oil 24 3.1.1 Natural Triglyceride and Wax Ester Base Oils 26 3.1.2 Mineral Oils 26 3.1.3 Synthetic Oils 30 3.2 Base Oil Viscosity and Density 41 3.2.1 Viscosity–Temperature 44 3.2.2 Viscosity–Pressure–Temperature 45 3.2.3 Density, Compressibility 47 3.3 Thickener 49 3.3.1 Soap Greases, Simple Greases 50 3.3.2 Complex Greases 51 3.3.3 Non-soap Thickeners 52 3.3.4 Mixed Thickeners 52 3.3.5 Mechanical Structure 53 3.3.6 Oil Retention 56 3.3.7 Properties of Different Types of Grease Thickeners 56 3.4 Additives 61 3.4.1 Corrosion Inhibitors 62 3.4.2 Antioxidants 62 3.4.3 EP/AW Additives 63 3.5 Solid Fillers/Dry Lubricants 66 3.5.1 MoS2 and Graphite 66 3.5.2 Nanoparticles 66 3.5.3 ZnO 66 3.5.4 Teflon (polytetrafluoroethylene) 66 3.5.5 Polyethylene 66 3.6 Compatibility 67 3.7 Polymer Grease 67 4 Grease Life in Rolling Bearings 71 4.1 Introduction 71 4.2 Relubrication Intervals and Grease Life 71 4.3 The Traffic Light Concept 72 4.3.1 Low Temperatures 74 4.3.2 Extreme Low Temperature 75 4.3.3 Extreme High Temperature 75 4.4 Grease Life as a Function of Temperature in the Green Zone 75 4.5 SKF Relubrication and Grease Life 76 4.6 Comparison Grease Life/Relubrication Models 78 4.7 Very Low and High Speeds 82 4.7.1 Speed Ratings and Speed Factors 82 4.7.2 High Speed 82 4.7.3 Very Low Speeds 85 4.8 Large Rolling Bearings 85 4.9 Effect of Load 86 Contents ix 4.9.1 Varying Load 86 4.9.2 Direction of Load 89 4.9.3 Very Heavy Loads 89 4.10 Effect of Outer-Ring Rotation 90 4.11 Cage Material 90 4.12 Bearing Type 91 4.12.1 Roller Bearings 91 4.12.2 Hybrid Bearings 91 4.13 Temperature and Bearing Material 92 4.14 Grease Fill 94 4.15 Vertical Shaft 95 4.16 Vibrations and Shock Loads 96 4.17 Grease Shelf Life/Storage Life 97 5 Lubricating Grease Rheology 99 5.1 Visco-Elastic Behaviour 99 5.2 Viscometers 102 5.2.1 Parallel Plate and Cone-Plate Viscometers 103 5.2.2 Errors in Rheometry Measurements 103 5.2.3 Errors in Thin Film Parallel Plate Rheometry Measurements 105 5.3 Oscillatory Shear 108 5.3.1 Theory 108 5.3.2 Application to Grease 110 5.3.3 Effect of Thickener Concentration 112 5.4 Shear Thinning and Yield 112 5.4.1 Grease 112 5.4.2 Lubricating Oil 116 5.5 Yield Stress 118 5.5.1 The Concept 118 5.5.2 Influence of Temperature 119 5.5.3 Consistency 120 5.6 Wall-Slip Effects 122 5.7 Translation Between Oscillatory Shear and Linear Shear Measurements 125 5.7.1 Viscosity 125 5.7.2 Yield Stress 126 5.8 Normal stresses 126 5.9 Time Dependent Viscosity and Thixotropy 128 5.10 Tackiness 133 5.10.1 Introduction 133 5.10.2 Tackifiers 134 5.10.3 Pull-Off Test 135 5.10.4 Other Tests 136 6 Grease and Base Oil Flow 137 6.1 Grease Flow in Pipes 137 6.1.1 Approximation Using the Newtonian Pipe Flow Equations 137 x Contents 6.1.2 Non-Newtonian Fluid 138 6.1.3 Bingham Rheology 139 6.1.4 Sisko Rheology 140 6.1.5 Power Law Rheology 140 6.1.6 Herschel–Bulkley Rheology 140 6.1.7 The Darcy Friction Factor 142 6.1.8 Transient Effects 144 6.1.9 Air in Grease 144 6.1.10 Entrance Length 145 6.1.11 Solid Particles in Grease Flow 145 6.1.12 Wall-Slip/Slip Layer 145 6.1.13 Impact of Roughness 147 6.1.14 Grease Aging in Pipes 149 6.2 Grease Flow in Rolling Bearings 149 6.2.1 Churning 149 6.2.2 Flow Through Bearing Seals 152 6.2.3 Relubrication 152 6.2.4 Grease Flow Around Discontinuities 153 6.2.5 Creep Flow 153 6.2.6 Flow Induced by Vibrations 155 7 Grease Bleeding 157 7.1 Introduction 157 7.2 Ball Versus Roller Bearings 158 7.3 Grease Bleeding Measurement Techniques 158 7.4 Bleeding from the Covers and Under the Cage 159 7.5 A Grease Bleeding Model for Pressurized Grease by Centrifugal Forces 161 7.5.1 Oil Bleeding Model 162 7.5.2 Quality of the Model 166 8 Grease Aging 171 8.1 Mechanical Aging 172 8.1.1 Softening of Grease in Rolling Bearings 172 8.1.2 Hardening of Grease in Rolling Bearings 179 8.2 Grease Oxidation 179 8.3 The Chemistry of Base Oil Film Oxidation 181 8.3.1 Chemical Reactions 181 8.4 Oxidation of the Thickener 183 8.5 A Simple Model for Base Oil Degradation 185 8.6 Polymerization 186 8.7 Evaporation 186 8.8 Simple Models for the Life of Base Oil 187 8.8.1 Booser’s Oil Life Model 187 8.8.2 Two Phase Model 188 Contents xi 9 Film Thickness Theory for Single Contacts 191 9.1 Elasto-Hydrodynamic Lubrication 192 9.1.1 History 192 9.1.2 The Navier–Stokes Equations 193 9.1.3 The Reynolds and Thin Film Equation 194 9.1.4 Cavitation 198 9.2 Contact Geometry and Deformation 198 9.2.1 Rigid